Chapter 13 Aromatic compounds

13.1 Introduction

1. How to find benzene?

2. Kekule(克库勒） was the first to recognize that these early aromatic compounds all contain a six-carbon unit.

Benzene 苯

13.2 Nomenclature of benzene Derivatives

F Cl Br NO2

Fluorobenzene Chlorobenzene Bromobenzene Nitrobenzene

(·ú±½£© £¨Âȱ½£© £¨äå±½£© Ïõ»ù±½£©

1. Monosubstituted benzenes µ¥È¡´ú±½

CH3 CH2CH3OH NH2

Methylbenzene Ethylbenzene Hedroxybenzene Aminobenzene

(¼×±½£© £¨ÒÒ±½£© £¨±½·Ó£© £¨°±»ù±½£¬ ±½°·£©

1. Monosubstituted benzenes µ¥È¡´ú±½

Toluene Phenol Aniline

Monosubstituted benzenes

SO3H COOH COCH3 OCH3

Benzenesulfonic acid Benzoic acid Acetophenone Anisole

(±½»ÇËᣩ £¨±½¼×Ëᣩ £¨±½ÒÒͪ£© £¨±½¼×ÃÑ£©

Br Br

Br

1,2-Dibromobenzene 1,3-Dibromobenzene 1,4-Dibromobenzene

(ÁÚ-¶þäå±½£©

Br

BrBr

o-Dibromobenzene m-Dibromobenzene p-Dibromobenzene

(¼ä-¶þäå±½£© (¶Ô-¶þäå±½£©

When two substituents are present, their relative positions are indicated by the prefixes ortho, meta, and para (abbreviated o-, m-, and p-) or by the use of numbers.

Nitrobenzoic acids 硝基苯甲酸

COOH COOH

COOH

2-Nitrobenzoic acid 3-Nitrobenzic acid 4-Nitrobenzic acid

(ÁÚ-Ïõ»ù±½¼×Ëᣩ

NO2

NO2 NO2

o-Nitrobenzoic acid m-Nitrobenzoic acid p-Nitrobenzoic acid

(¼ä-Ïõ»ù±½¼×Ëᣩ (¶Ô-Ïõ»ù±½¼×Ëᣩ

The dimethylbenzenes are called xylenes (二甲苯）

CH3 CH3

CH3

1,2-Dimethylbenzene 1,3-Dimethylbenzne 1,4-Dimethylbenzene

(ÁÚ-¶þ¼×±½£©

CH3

CH3 CH3

o-xylene m-xylene p-xylene

(¼ä-¶þ¼×±½£© (¶Ô-¶þ¼×±½£©

If more than two groups are present on the benzene ring,

their positions must be indicated by the use of

numbers.CH3 Cl

Br

1,2,3-Trimethylbenzene 1,2,3-Trichlorobenzne 1,2,4-Tribromobenzene

(1,2,3-Èý¼×±½£©

CH3

ClBr

(1,2,3-ÈýÂȱ½£© (1,2,4-Èýäå±½£©

CH3

Cl

Br

(not 1,3,4-tribromobenzene)

The new parent name is used

COOHSO3H

3,5-Dinitrobenzoic acid 2,4-Difluorobenzenesulfonic acid

(3,5-¶þÏõ»ù±½¼×Ëᣩ (2,4-¶þ·ú±½»ÇËᣩ

NO2O2N

F

F

When the C6H5- group is named as a substituent, it is calle

d a phenyl group (ph-).

CH2CH2CH2CH3 CH3C=CHCH3

ph

CH3CHCH2CH2CH2CH2CH3

ph

Butylbenzene 2-Phenyl-2-butene 2-Phenylheptane

¶¡»ù±½ 2-±½»ù-2-¶¡Ï© 2-±½»ù¸ýÍé

The phenyl group is often abbreviated as C6H5-, Ph-.

The name benzyl is an alternative name for the phenylmethyl gr

oup:

CH2-

Phenyl

Benzyl CH2Cl Benzyl chloride

Cl Chlorobenzene

ÜлùÂÈ

±½»ù

Üлù

13.3 Reactions of benzene

Benzene

Br2 / CCl4

dark, 25oCNo addition of bromine

KMnO2 / H2O

25 oC

H2O, HCl

heat

H2 / Ni

No hydration

No oxidation

Slow additionat hight temperatureand pressure

Why?

Why?

Why?

Why?

Br2 FeBr3

Br Br

Br

Not observedObserved

Why?

13.4 The Kekule Structure for benzene 苯的克库勒结构

In 1865， Kekule, the originator of the structural theory, proposed the first definite structure for benzene, a structure that is still used today. Kekule suggested that the carbon atoms of benzene are in a ring, that they are bonded to each other by alternating single and double bonds, that one hydrogen atom is attach to each carbon ato

m.

The Kekule formula for benzene

苯的克库勒式

orC

C

C

C

C

CH

H

H

H

H

H

C

C

C

C

C

CH

H

H

H

H

H

C

C

C

C

C

CH

H

H

H

H

H

¿Ë¿âÀյĹ²Õñ½á¹¹Ê½

¿Ë¿âÀյĹ²Õñ½á¹¹Ê½

Br

Br

Br

Br

No isomer

Same compoundͬһÎï

13.5 The stability of benzene苯的稳定性

+

+ 2H2

+ 3H2

Pt

Pt

Pt

Benzene

1,3-Cyclohexadiene

Cyclohexene Cyclohexane

Ho = - 28.6 kcal mol -1

Ho = (2X- 28.6)= -57.2 kcal mol -1Calculated

ObservedHo = - 55.4 kcal mol -1

Ho = (3X- 28.6)= -85.8 kcal mol -1Calculated

ObservedHo = - 49.8 kcal mol -1

Difference 36.0 kcal mol -1

H2

Fig. Relative stabilities 相对稳定性

Benzene

1,3-Cyclohexadiene

Cyclohexane

Ho = - 28.6 kcal mol -1 Ho = - 55.4 kcal mol -1

Ho = (3X- 28.6)= -85.8 kcal mol -1Calculated

ObservedHo = - 49.8 kcal mol -1

Difference 36.0 kcal mol -1

(¹²ÕñÎȶ¨»¯ÄÜ£©

Pot

enti

al e

ner

gy

+ H2

+ 2H2

+ 3H2

13.6 Modern theories of the structure of benzene

13.6A the resonance explanation of the structure of benzene (苯环结构的共振理论解

释）

Benzene Cyclohexane

Ho = (3X- 28.6)= -85.8 kcal mol -1Calculated

ObservedHo = - 49.8 kcal mol -1

Difference 36.0 kcal mol -1

(¹²ÕñÎȶ¨»¯ÄÜ£©

+ 3H2

13.6B The molecular orbital explanation of the structure of benzen

e （苯结构的分子轨道理论解释）

H

H

H

H

H

H

120 o120 o

1. sp2-hybridized

2. Angel is 120o.

3. Planar (ËùÓеÄÔ ×Ó¹²Æ½Ã棩

Fig. 13.3 Overlapping p orbitals in benzene (苯的 p轨道的重

叠）

Antibonding MO

Bonding MO

·´¼ü¹ìµÀ

³É¼ü¹ìµÀ

Fig. 13.4 How six p atomic orbital combine to form six pi- molecular orbitals

Fig. 13.5 Shapes of the pi-molecular orbitals of benzene as viewed from down

Two nodes

One node

Three nodes

13.7 HUCKEL’S Rule(休克尔规则） : The (4n+2) pi-Electron Ru

le

Conditions;

1. The (4n+2) pi-electron， where n=1,2,3---

2. Planar monocyclic ring conjugate system (平面单环共轭体系）

符合这两个条件的环具有芳香性（ Aromatic), 具有芳香性的物质，是稳定的。化学

性质表现出难加成，易取代。

For example

pi-electrons = 4 + 2 = 6 n=1

(Aromatic ·¼ÏãÐÔ£©

Benzene

pi-electrons = 8 didn't follow 4n+2 rule

(No Aromatic ²»¾ßÓз¼ÏãÐÔ£©

Cyclooctatetraene£¨»·ÐÁËÄÏ©£©

It is not coplanar

13.7A The Annulenes （轮烯）

The name annulene has been proposed as a general name for monocyclic compounds that can be represented by structures having alternating single and double

bonds.

Benzene Cyclooctatetraene£¨»·ÐÁËÄÏ©£©([6] annulene)

[6]-ÂÖÏ© ([8] annulene)[8]-ÂÖÏ©

14

[14] Annulene

pi-electrons is 144n+2 = 4X3+2 = 14 (·ûºÏ4n+2)(aromatic)(¾ßÓз¼ÏãÐÔ£©

18

[18] Annulenepi-electrons are 184n+2 = 4X4+2 = 18 (·ûºÏ4n+2)(aromatic)(¾ßÓз¼ÏãÐÔ£©

16

[16] Annulenepi-electrons is 164n+2 = 4X3+2 = 14(²»·ûºÏ4n+2)(not aromatic)(²»¾ßÓз¼ÏãÐÔ£©

[10] Annulene

pi-electrons is 104n+2 = 4X2+2 = 10 (·ûºÏ4n+2), Do not obey Huckel's Rule(not aromatic)(²»¾ßÓз¼ÏãÐÔ£©It is not planar (²»¹²Æ½Ã棩

HH

[10] Annulene [10] Annulene

None is aromatic because none is planar

It is not planar (²»¹²Æ½Ã棩, (²»¾ßÓз¼ÏãÐÔ£©

Cyclobutadiene is a 4n molecule not a 4n+2 molecule, and as we would expect, it is a highly unstable compound and it is not a

romatic.

Cyclobutadieneor [4] annuleneDo not obey HUCKEL's Rule(not aromatic)

13.7B Aromatic Ions 芳香离子

Cyclopentadiene is not aromatic; however, it is unusually acidic for a hydrocarbon. Because of its acidity, cyclopentadiene can be converted to its anion by treatment with moderat

ely strong bases.

H H

Cyclopentadiene»·Îì¶þÏ©

Strong

Base

H

_

Cyclopentadienyl anion»·Îì¶þÏ©¸ºÀë×Ó

+ H+

H

_

Cyclopentadienyl anion»·Îì¶þÏ©¸ºÀë×Ó

ÊÇ·ñ¾ßÓз¼ÏãÐÔÐÔ£¿

H H

Cyclopentadiene»·Îì¶þÏ©

Strong

Base _

Cyclopentadienyl anion»·Îì¶þÏ©¸ºÀë×Ó

+ H+

sp3sp2

pi-electrons are 6, follow HUCKEL'RuleIt is aromatic compoundIt is not aromatic compound

Cycloheptatriene (环庚三烯）

H H

- H-

+

Cycloheptatriene»·¸ýÈýÏ©(it is not aromatic)

Cycloheptatrienyl cation»·¸ýÈýÏ©ÑôÀë×Ó(It is aromatic)

+

13.8 Other aromatic compounds

13.8A benzenoid aromatic compounds (苯形芳香化合物）

Representatives of one broad class of aromatic compounds called polycyclic benze

noid aromatic hydrocarbons.1

2

3

45

6

7

8

NaphthaleneÝÁ

1

2

3

410

9

6

5

7

8

AnthracneÝì

7

8

9

101

2

3

4

Phenanthrene·Æ

6

5

2 5

4

10

6

3

7

8

Pyrene

ÜÅ

1

9

According to resonance theory, a molecule of naphthalene can be considered to be a hybrid of thre

e Kekule structure.

Naphthalene

Fig. 13.2 The p orbitals of naphthalene

Naphthalene

=

Naphthalene

( Aromatic ¾ßÓз¼ÏãÐÔ)

Pyrene ÜÅ

£¨·¼ÏãÐÔ£©

[14]-Annulene

( no aromatic ²» ¾ßÓз¼ÏãÐÔ) H3C CH3

trans-15,16-Dimethyldihydropyrene( aromatic ¾ßÓз¼ÏãÐÔ)

13.8B Nonbenzenoid aromatic compounds非苯型的芳香族化合

物

H3C CH3

trans-15,16-Dimethyldihydropyrene( aromatic ¾ßÓз¼ÏãÐÔ)

Azulene

°Â

The dipole moment is 1.0D

(aromatic)

13.9 Reduction of aromatic compounds: The Birch reduction 伯

奇还原

Benzene

H2 / Ni

Slow+

Cyclohexadiene

H2 / Ni

fast

H2 / Ni

fast

Cyclohexene Cyclohexane

13.9A The Birch reduction 伯奇还原

Benzene can be reduced to 1,4-cyclohexadiene by treating it with an alkali

metal(sodium (Na), lithium (Li), or potassium (K)) in a mixture of liquid ammonia and

alcohol.

Benzene

NH3, EtOH

1,4-Cyclohexadiene

Na or K

The Mechanism of Birch reduction

伯奇还原的机理

Benzene

NH3, EtOH

Benzene anion radical

Na .

:-

.

-:

.

EtOH

H

H

.

H

H

.

Cyclohexadienyl radical»·¼º¶þÏ©×ÔÓÉ»ù

Na .

H

H

:-

H

H

:-

Cyclohexadienyl aniom»·¼º¶þÏ©¸ºÀë×Ó

EtOH

H

H

H

H

1,4-Cyclohexadiene

Dissolving metal reductions of this type were developed by the Australian chemist A.J. Birch

and have come to be known as Birch reductions

Methoxybenzene±½¼×ÃÑ

NH3, EtOH

LiOCH3 OCH3

1-Methoxy-1,4-cyclohexadiene 80 %

Reduction of 1,2-dimethylbenzene (o-xylene 邻 -二甲苯 )

1,2-Dimethylbenzene

NH3, EtOH

NaCH3 CH3

1,2-Dimethyl-1,4-cyclohexadiene 77-92 %

CH3 CH3

(o-xylene)

Birch reduction of sodium benzoate

Sodium benzoate

NH3, EtOH

Na

(±½¼×ËáÄÆ)

COONa COONa

H3O+

COOH

89-52 %

13.10 Benzylic radicals and cations

Removal of a hydrogen atom from the methyl group of methylbenzene (Toluene) produces a radical called the benzyl radical

(苄基自由基）HH

H

Methylbenzene(Toluene)

HH

The benzyl radicalÜлù×ÔÓÉ»ùStable

.

-e-

H

H+

Benzylic cationÜлùÑôÀë×ÓStable

Benzylic radicals and benzylic cations are conjugated unsaturated systems and both are stable.

This exceptional stability of benzylic radicals and cations is easily e

xplained by resonance theory.

HH

The benzyl radicalÜлù×ÔÓÉ»ùStable

.

.

H H

.

H H

.

H H

Benzylic radicals are stabilized by resonance

Benzylic cations are stabilized by resonance

HH

Benzylic cation ÜлùÑôÀë×ÓStable

+

+

H H

+

H H

+

H H

13.11 Allylic and benzylic halides in nucleophilic substitution r

eactions (亲核取代反应中的烯丙基卤和苄

基卤）（不讲）

13.12 Heterocyclic aromatic compounds (杂环芳香族化合

物）Heterocyclic compounds containing nitrogen, oxygen, or sulfur are by far the most common. Four important examples are given here in their KeKule forms. These four com

pounds are all aromatic.

Pyridine

NNH:

:

1

2

3

4

5

61

2

34

5

Pyrrole

ßÁठßÁ¿©

O S:

: ::

1

2

34

5

1

2

34

5

Furan Thiophene

߻ૠàç·Ô

The nitrogen atoms in molecules of both pyridine and pyrro

le are sp2 hybridized.

Pyridine

:

ßÁà¤

NN

Fig.13.15 The orbital structure of pyridine weak base (Èõ¼î£©

NH

:

Pyrrole

ßÁ¿©

H

Fig.13.16 The orbital structure of pyrrole

13.17 The orbital structures of furan and thiophene

O::Furan

߻ૠFig.13.16 The orbital structure of Furan

S ::Thiophene

àç·ÔFig.13.16 The orbital structure of thiophene

The oxygen and sulfur atoms of furan and thiophene carry an Unshared pair of electrons in an sp2 orbital that is orthogonal to the πsystem.

13.13 Aromatic compounds in biochemistry

Two amino acids necessary for protein synthesis contain the benzene

ring:

CH2CHCOO-

NH3+

Phenylalanine±½±û°±Ëá

CH2CHCOO-

NH3+

TyrosineÀÒ°±Ëá

HO

CH2CCHOO-

NH3+

TryptophanÉ«°±Ëá

NH

N

NNH

N

PurineàÑßø

1

2

34

56 7

8

9

NH

IndoleßÅßá

N

N

Pyrimidineà×à¤

1

2

3

4

5

6

Homework P 548

Additional Problems13.22, 13.23, 13.28